Add an erlang-mk target for automated updating of erlang.mk
[inaka/elvis#96] Elvis plugin, updated README.
Update Ranch, Cowlib and Cowboy versions
Use erlang:halt/0 for c_src.mk

* erlang:halt/0 is much faster than init:stop/0 to terminate
  execution (no wait time)
Support for compiling SNMP MIBs

This is unfortunately unsuited as a plugin due to its dependence on erlc
for compilation. The MIBs must be compiled before an Erlang source.
Add support for excluding erl files
Add package hanoidb to the index
Don't try to compile mibs if the mibs/ directory is missing
Add erl_interface for c_src compile and link

Using LDFLAGS, LDLIBS for link
Don't try compile c_src if not exist
Compile all *.c files in c_src and sub dir
Make env.mk delete on distclean

Because env.mk creates a considerable time, then delete it less often.
Add package unicorn to the index
Add package jiffy_v to the index
Enabled the c_src plugin by default
plugin/c_src: compile C++ source

Add compile .cc,.cpp,.C files as C++.
Compile each source file to .o.
Link all .o object to .so.
Delete .o on clean target.
plugin/c_src: remove C_SRC_OPTS

We can just LDFLAGS += and whatnot.
Update README for C/C++ compiler
Add missing variable to the README
add escript plugin